Haishan Bu

Nucleation mechanism and crystallization behavior of poly(ethylene terephthalate) containing ionomers

By means of differential scanning calorimetry, the influences of AClyn® on the crystallization of poly(ethylene terephthalate) (PET) were investigated and compared with Surlyn®. AClyn initiates heterogeneous nucleation and substantially increases the crystallization rate of PET while Surlyn dissolves in molten PET and reacts with PET chains, producing ionic chain ends. They subsequently associate into ionic clusters which, eventually, act as the true nucleation species. AClyn is more effective to accelerate nucleation than Surlyn. Crystallization kinetics analysis by the Avrami equation indicates that an athermal nucleation seems to occur in the case of both AClyn and Surlyn.

Fast crystallization of liquid crystalline copolyesters based on poly(ethylene terephthalate)

Crystallization of a series of liquid crystalline copolyesters prepared from p-hydroxybenzoic acid (HBA), hydroquinone (HQ), terephthalic acid (TA), and poly(ethylene terephthalate) (PET) was investigated by using differential scanning calorimetry (DSC). It was found that these copolyesters are more crystalline than copolyesters prepared from PET and HBA. Insertion of HQ-TA disrupts longer rigid-rod sequences formed by HBA and thus enhances molecular motion and increases the crystallization rate. The effects of additives on the crystallization of the copolyesters were also studied. Sodium benzoate (SB) and sodium acetate (SA) increase the crystallization rate of the copolyesters at low temperature, but not at high temperature. It is most likely that liquid crystalline copolyesters do not need nucleating agents, and small aggregates of local-oriented rodlike segments in nematic phase could act as primary nuclei. Chain scission of the copolyesters caused by the reaction with the nucleating agents was proved by the determination of intrinsic viscosity and by the IR spectra. Diphenylketone (DPK) was shown to effectively promote molecular motion of chains, leading to an increase in the crystallization rate at low temperature, but it decreased the crystallization rate at high temperature.

Structure of single-molecule single crystals of isotactic polystyrene and their radiation resistance

Based on the one-band tight-binding model, we systematically investigate the interlayer exchange coupling and the angular dependence of the coupling energy in a magnetic sandwich covered on both sides by nonmagnetic films. Our results show that (i) the thickness of magnetic and outer nonmagnetic films influence significantly the oscillatory behavior of exchange coupling, (ii) the appearance of noncollinear exchange coupling is very sensitive to the thickness of magnetic and outer nonmagnetic layers, (iii) the nonoscillatory component of the coupling varies generally with the thickness of magnetic or outer nonmagnetic films, and (iv) the results in the case where the thickness of both magnetic or both outer nonmagnetic films vary simultaneously are significantly different from that in the case where the thickness of one of the two magnetic or outer nonmagnetic films is fixed while the other is varied. These results are qualitatively in agreement with the experimental measurements

A unique morphology of freeze-dried poly(ethylene oxide) and its transformation

Freeze-dried poly(ethylene oxide) (PEO) samples were prepared from a 1 x 10(-2) wt.% solution in benzene. After isothermal crystallization at 318.2 +/- 0.1 K, a unique spherulite-like morphology with special inner structure was observed by transmission electron microscope. The crystal structure was identified to be the triclinic form. The molecular chains in it adopt the planar zigzag conformation, which was only found in the sample under tension. Howe-ver, it was revealed in this paper that the triclinic crystals formed in the freeze-drying process can exist at room temperature for a long period. After being heated at a temperature far above the equilibrium melting point of PEO and cooled down to 318.2 +/- 0.1 K, they transformed to the common monoclinic form. The resulting crystals show a great variety of regular polygonal shapes with multiple twin patterns

Crystallization and Melting Behavior of Nanopolymeric Particles Containing Single or a Few Chains

Samples of nanopolymeric particles, each containing a single chain or a few chains, were prepared by a freeze-drying method from dilute solutions of isotactic polystyrene (iPS) in benzene. Thermal analysis of the particle samples revealed that the cold crystallization temperature greatly decreases and the crystallization rate remarkably increases as the solution concentration decreases. The increase in crystallization rate can be attributed to fewer interchain entanglements within and between particles, thus proving experimentally for the first time that entanglements can be a large barrier to the crystallization of polymers. Annealing of the particle samples at 373.2 K slightly changed the crystallization behavior, indicating that the interdiffusion of chains between these particles is rather sluggish in the vicinity of T g of bulk iPS. Crystallization is assumed to be accomplished before the chain interdiffusion, and nanocrystals form in situ within compact single- and few-chain globules. The average size of crystals is larger than that of single-chain crystals because the single-chain crystals may initiate other chains to crystallize on the growing crystal surface. Meanwhile, we also found that the nanocrystals have a lower melting temperature than the bulk polymer as a result of their small size.

Single-molecule single crystals of poly(ethylene oxide)

Single-molecule single crystals were prepared from two fractions of poly(ethylene oxide) (PEO) with narrow molar mass distribution and an equimolar mixture of the two fractions. It was proven that the molar mass distribution of the single-molecule single crystals from the mixed sample corresponds to an addition of those of the pure fractions. Well-shaped crystals were obtained after isothermal crystallization or on annealing. A variety of morphologies typical for multimolecule single crystals of PEO were found and are described on the basis of the various known modes of twinning. The results are in agreement with the known unit cell of PEO.

INFLUENCE OF ADDITIVES ON CRYSTALLIZATION OF POLY(ETHYLENE 2,6-NAPHTHALATE)

The influences of additives, Ceraflour 991® (low molar mass polyethylene), Ceraflour 993® (low molar mass polyamide), and poly (1,4-butylene sebacate), on the crystallization of poly(ethylene 2,6-naphthalate) (PEN) were investigated by means of differential scanning calorimetry, scanning electron microscopy, and polarized optical microscopy. It was revealed that these additives could substantially accelerate the crystallization of PEN at low temperature due to an improvement of molecular motion of PEN. They can be attributed to a kind of nucleating promoter. They also enhance the crystallization rate of PEN at high temperature; this could be caused by phase separation between additive and PEN. The separated microdroplets of the additives serve as heterogeneous nuclei to initiate primary nucleation. In the high temperature region, however, Ceraflour 993 is much more effective to accelerate nucleation than others, and it acts as if it is also a nucleating agent.

Solid‐State Polymerization of a Liquid Crystalline Copolyester Derived from 2,6‐Naphthalene Dicarboxylic Acid, Terephthalic Acid, 4‐Acetoxybenzoic Acid and Hydroquinone Diacetate

Solid‐state polymerization (SSP) has been applied for the synthesis of a wholly aromatic thermotropic liquid crystalline polymer derived from 4‐acetoxybenzoic acid, hydroquinone diacetate, 2,6‐naphthalene dicarboxylic acid, and terephthalic acid. The influence of various operating parameters, such as reaction temperature, reaction time, nitrogen gas flow velocity, and particle size, on the solid‐state polymerization has been studied, and the SSP products were characterized by polarized light microscope (PLM), differential scanning calorimeter (DSC), viscosity measurement, and nanoindentation test. It was found that the molar mass of the copolyester substantially increased during the SSP process, as shown by measurements of melting temperature and inherent viscosity of the product. The SSP reaction mechanism and rate are discussed with the changes of the operating parameters. Nanoindentation test shows the increase of hardness and modulus of the copolyesters, which is due to the increase of molar mass. A banded texture of the copolyesters with narrow bands was observed after SSP by using PLM.

Unique thermal behaviors of collective single and few-chain compact globules of isotactic polystyrene

Collective single- and few-chain compact globules of isotactic polystyrene (i-PS) were prepared from very dilute solution via the freeze-drying technique. A small exothermic peak was found in the differential scanning calorimetry (DSC) traces of freeze-dried samples. This is a manifestation of the existence of ‘cohesional entanglements’.

Unique morphologies found in freeze-dried poly(ethylene oxide) prepared from dilute solutions

Freeze-dried particles of poly(ethylene oxide) (PEO) were prepared from sublimation of a 1 × 10 -2 wt.-% solution of PEO in benzene in an ice-salt bath. After isothermal crystallization at 318.2 ± 0.1 K for 2 h, an unusual planar zigzag form of PEO was found. A variety of unique spherulite-like morphologies were also observed, and their formation is discussed on the basis of the interaction between solvent molecules and segment of the polymer chains.